1. Field of the Invention
Embodiments of the present invention generally relate to a method and apparatus for processing substrates, such as solar panel substrates, flat panel substrates, or semiconductor substrates, using plasma. More particularly, embodiments of the present invention relate to a radio frequency (RF) current return path for a plasma processing chamber.
2. Description of the Related Art
Plasma enhanced chemical vapor deposition (PECVD) is generally employed to deposit thin films on substrates, such as semiconductor substrates, solar panel substrates, and liquid crystal display (LCD) substrates. PECVD is generally accomplished by introducing a precursor gas into a vacuum chamber having a substrate disposed on a substrate support. The precursor gas is typically directed through a gas distribution plate situated near the top of the vacuum chamber. The precursor gas in the vacuum chamber is energized (e.g., excited) into a plasma by applying a radio frequency (RF) power to the chamber from one or more RF sources coupled to the chamber. The excited gas reacts to form a layer of material on a surface of a substrate that is positioned on a temperature controlled substrate support. The distribution plate is generally connected to a RF power source and the substrate support is typically connected to the chamber body providing a RF current return path.
Uniformity is generally desired in the thin films deposited using PECVD processes. For example, an amorphous silicon film, such as microcrystalline silicon film, or a polycrystalline silicon film is usually deposited using PECVD on a flat panel for forming p-n junctions required in transistors or solar cells. The quality and uniformity of the amorphous silicon film or polycrystalline silicon film are important for commercial operation. Therefore, there is a need for PECVD chambers with improved plasma and deposition uniformity.
As the demand for larger LCD's and solar panels continues to grow, so does the size of the substrate that is used to make the LCD's and solar panels. The size of the substrates now routinely exceeds 1 square meter in area. When compared to the size of semiconductor substrates, which typically are about 300 millimeters in diameter, it can be easily understood that a chamber sized to process a semiconductor wafer may not be sufficiently large to process a substrate of 1 square meter or larger. Thus, larger area processing chambers need to be developed.
These large area processing chambers may, in some cases, be identical to the semiconductor counterpart chambers where simply scaling up in size achieves acceptable results. In other cases, scaling up the size of the processing chamber is not effective, as unforeseen difficulties occur when scaling up the processing chambers. Designing large chambers for application of RF energy is one example where scaling does not produce satisfactory results.
Additionally, the process conditions for processes that are performed in the large area processing chambers may need to be adjusted. Determining proper gas flows, timing sequences, RF power application, temperature conditions, and other process variables may require a significant amount of research and experimentation that is far beyond routine.
Therefore, care needs to be taken to design a chamber that can process large area substrates.